developments in low-temperature metal-based packaging · 2011-12-15 · • project & research...

IEEE Santa Clara Valley Chapter, Components, Packaging and Manufacturing Technology Society

December 14, 2011

www.cpmt.org/scv

Jiyoung Chang and Liwei LinPh.D. Candidate, Department of Mechanical Engineering

University of California at Berkeley

Developments in low-temperature metal-based packaging

2011. 12.14

11

22

Contents

• Project & Research goals• Low temperature metal packaging

– Dry thermocompression– Rapid Thermal process– Solder pre-reflow– Insertion of thin metal layer

• Summary


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Research goals

– Flux-less bonding

– Short bonding time & Low bonding temp.

– Sub-100m width bonding ring→ Photolithography definable & Electroplating

– Hermetic sealing→ Low permeability required

– CMOS compatible process

Center for Micro/Nano Scaling Induced Physics (MiNaSIP) in U.C.Berkeley- Subgroup research with Freescale™ semiconductor

44

Material selection

• Material Selection Criteria– Temperature– Deposition & bonding method– Permeability – Mechanical properties

*Glass Frit : Popular in MEMS packaging Deposition : Screen-printing (min.w≈150µm)

Temperature : 450 ˚C

Time :15-30 minutes

Mechanical : Brittle

→ Metal bonding Silicon

MEMSGlass frit

cap

F


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Dry Thermo-Compression (SnCu-Cu)

• 227 > Tbond > 232

• Results:– No bond or Solder reflowed

• Low process controllability – Non-planar surface issue– Surface oxidation may prevent

inter-diffusion

t=5m

T

t

F

Temperature profile Applied force profile

t=13m

t=13mt=5m

t

Bond viewed through pyrex cap

500 µm

200 µm

< Flipchip bonding parameters >

66

Sn Reflow Bonding via RTP

Silicon

MEMSCMOS Cu

capSn

Sn

Silicon

MEMSCMOS Cu

cap

• RTP (Rapid Thermal Process)• Short bonding time with high

temperature ramping up speed to prevent solder’s squeezing out

• Solder reflow benefitsLess sensitive to surface non-uniformitiesLow force


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Cap wafer fabrication Process Flow

Metallization(Ti 250Å / Ni 2000Å)

Lithography

Electroplate(Sn : 4-6m)

Etchback

Cap Wafer

Cap Wafer

Sn

Photoresist

Sn

metallization

88

Device substrate

Photoresist

metallizationMetallization(Ti 250Å / Ni 2000Å)

Lithography

Electroplating(Cu alloy, 4-8 µm)

Strip resist &Etch back

Substrate wafer fabrication Process Flow

Cu

Cu


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RTP Bonds – Minimal Thermal Budget

• Thermal budgeting compared to glass frit– Time reduction – 10-50X – Temperature reduction – 1.5-2X

200 µm

Temp.

Timehold

ramp

Tmax =250-300 ˚Cramp = 10 secondshold = 10 seconds

1010

Mechanical strength comparable with glass frit

Pull-Test Setup

InstronArm

package

F (measured)

pull-tested bond

Material System Bond Strength(MPa)

Glass frit (thermo-compression)

8.5

Solder reflow (RTP) 7.5


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Leak & Autoclave Test• Dyed-IPA leak test

– Gross-leak detection– Small surface tension of IPA allows quick permeation

through voids

• 12-hour Autoclave test failed : 125ºC & 24psi

Dyed-IPA

Bonding ring

Sealed,No dyed-IPA

1212

Why hermetic test failed??

- Formation of leakage pass-through due to…• Void formation

– Gas trapping during bonding process– Metal oxide on surface prevents reflow of solder

• Non-uniform bonding along the bonding ring– Lack of uniform pressure applied during bonding

→ Pre-reflow process approach


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Pre-Reflow process

♦ Purpose of Pre-reflow process• Break the surface oxide layer and reflow fresh solder• To minimize gas trapping which causes void formation in bonding

area

Without Pre-Reflow With Pre-Reflow

Gas Trapping! Small initialcontact area!

1414

Pre-reflow bonding concept


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Device fabrication

1616

Pre-reflow profile optimization

• Process parameters in typical solder ball reflow process– Temperature gradient in preheat zone

– Soak temperature and time when using Flux

– Temperature gradient from soak to maximum temperature

– Maximum peak temperature

– Cooling gradient in cooling zone

– Total heating time

• Current solder reflow profile is optimized for solder ball to form sphere shape not for square line form solder

→ Reflow profile needs to be modified for line shape bonding


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Typical solder ball reflow profile

• Typical solder ball reflow profile– SnAgCu & SnPb case– ≈ 20ºC~30ºC higher peak temperature than melting temperature– ≈ 40sec of time above melting temperature– Total of 3~5min of process

• Denis Barbini et al. “Process Considerations for optimizing a reflow profile”, SMT/July 2005, www.smtmag.com

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SnAg solder with Pre-reflow test

• Electroplated Sn(96.5)Ag(3.5) eutectic solder• Bonding ring dimension: Width ≈200m, Height ≈6m

Before reflow After reflow


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SnCu eutectic solder with Pre-reflow test

• Electroplated Sn(99.3)Cu(0.7) eutectic solder• Bonding ring dimension: Width ≈ 200m, Height ≈5m

Before reflow After reflow

2020

SnCu solder with multiple reflow processes

2nd Reflow 3rd Reflow


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• Electroplated 10m and 20m width bonding ring • Width to Height ratio - 1.5:1~2:1

Low Width to Height ratio bonding ring

Electroplated SnAgwith 10m width bonding ring

Electroplated SnCuwith 20m width bonding ring

2222

SnCu reflow test results

• Reflow profile modification• Formed half-cylinder after reflow• Partially non-uniform clogging surfaces observed

Electroplated SnCuwith 20m bonding ring

After reflow

Reflow


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SnAg 10m width ring pre-reflow results

• Reflow profile optimized in RTP• Uniform half-cylinder shape achieved• No clogging or solder gathering was observed

Electroplated SnAgwith 10m width bonding ring After optimized reflow

Reflow

2424

SnAg 10m width ring pre-reflow obtimization


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SEM image of SnAg bonding ring after pre-reflow

Reflow

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SnAg reflow results

• Surface profile measurement by micro profiler(ASIQ®)• Half-cylinder shape formation after reflow

0

2

4

6

8

10

35 40 45 50 55 60 65 70

Before Reflow

After Reflow

0

2

4

6

8

10

35 40 45 50 55 60 65 70

Before Reflow

After Reflow

Relative bonding ring position (m)

Hei

gh

t (

m)

SnAg solder reflow results plot


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Height uniformity measurement

2

3

4

5

6

7

8

9

1 3 5 7 9 11 13 15

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Flipchip bonding result

• Optimized reflow profile in pre-reflow was applied in Flipchip

• Z-height control to push the solder after pre-reflow

material Width Height

Substrate SnAg solder ≈10m ≈7~8m

Cap Ni 150m ≈200nm

Substrate

Cap1. Heating

2. Solder pre-reflow

3. Bonding with z-control


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Leak test result

Dyed IPA

Dyed IPADeviceArea

Bondingring

Dyed IPA

DeviceArea

Bondingring

Destructive evaluationof interface

Cap wafer

Substrate wafer

DeviceArea

A

B

C

3030

Hermeticity test• 12-hour Autoclave test failed

Cap

Substrate

• Why failed?-Too small bonding ring overlap-Mis-alignment and parallel adjustment problem in Flipchip

Destructive evaluation


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Thin metal layer insertion approach

• Alloy formation b/w bonding metal (Al)

• Stronger bonding than pure Al-Al bondingat same bonding condition

Metal #1

Metal #1

Metal #1

Metal #1

Thin metal #2

Substrate

Metal #1Thin metal #2

Thermo-compression bonding

3232

Temperature/pressure condition

60 70130 930 1000

100ºC

350ºC

1.5Kg

60 1000

Time(sec)

Time(sec)

Temperature

Pressure

• Lower bonding pressure (15MPa)

• Lower bonding temperature(350ºC, Tm(Al)=660ºC)


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Pull Test results

< Al to Al bonding > <Al+Sn20nm to Al bonding>

Adhesion layer on wafer (same color)

Bottom top Bottom top

-Same bonding parameter was applied for both cases

-Strong bonding at the contact interface for <Al+Sn20nm to Al bonding>

- No strong bond was made for Al to Al bonding case

Width : 50m

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Summary• Proof-of-Concept : Metal bonding for possible replacement of glass-frit

– CMOS-integrated process

– Compatible mechanical strength with glass-frit

• Rapid Thermal bonding result(no pre-reflow)– Successful leak-test for 200m bonding ring

– Failed 12hr hermetic test

• Solder reflow profile optimization for sub-100m metal bonding solution– Reflow profile optimization for 10mm SnAg boding ring

– Uniform half-cylinder shape reflow results

• Solder pre-reflow bonding of SnAg 10m bonding ring using Flipchip– Optimized reflow profile used for bonding

– Possibility of sub-100m bonding ring in MEMS packaging

• Metal insertion approach– Al-Sn-Al approach showed better bonding quality than pure Al-Al bonding

– Lower temperature and pressure


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Acknowledgement

• IEEE-CPMT

• Prof. Liwei Lin

• Mr. Kedar Shah

• Sponsor– Freescale™ semiconductor

– DARPA

Thank you!!

developments in low-temperature metal-based packaging · 2011-12-15 · • project & research...

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